Local Optoelectronic Characterization of Solvent-Annealed, Lead-Free, Bismuth-Based Perovskite Films.

Traditional organolead-halide perovskite-based devices have shown rapid improvements in their power conversion efficiency in less than a decade, yet challenges remain for improving stability and film uniformity, as well as the elimination of lead to address toxicity issues. We fabricated lead-free methylammonium bismuth iodide (MBI) perovskite films and studied the effect of solvent annealing with dimethylformamide (DMF) on both (1) the crystallinity and structure of the films with X-ray diffraction and scanning electron microscopy and (2) the local optoelectronic properties of the films as measured via (photo)conductive atomic force microscopy. We found that solvent annealing leads to improved crystallinity and increased grain size in the MBI films as compared to the thermally annealed films. Furthermore, solvent-annealed MBI films show significantly increased electrical conductivity in the out-of-plane direction. Photoconductivity in both solvent-annealed and thermally annealed MBI films was increased in the grain interiors versus the grain boundaries. It was observed that DMF-induced solvent annealing impacts charge transport through the film, which can be a unique design parameter for optimizing local optoelectronic properties. By studying how solvent annealing affects the MBI film structure and changes the ways in which charges are transported through the film, we have developed a better understanding of how local optoelectronic properties are affected by DMF annealing.